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You could just leave it out or use white wine. It won't taste the same, of course, but it will still be delicious.
Pegeen is a trusted home cook.
Aside from white or red wine, dry vermouth, cognac or brandy would work.
pierino is a trusted source on General Cooking and Tough Love.
I do use cognac myself but only in small amounts, like just one of those mini-bottles. The key to a good onion soup is taking a long time with the onions.
amysarah is a trusted home cook.
Ditto the suggestions above; or, if you'd like a flavor more related to sherry, madeira works well (small world, I have some in the fridge right now, made that way. Pretty tasty, if I do say so myself ;-)
To maximize the flavor of the onions, I would stick with dry white vermouth.
June is a trusted source on General Cooking.
My suggestion of a substitute would depend upon why you asked for one. Are you trying to avoid alcohol? or just want to know if you can sub another bevvie you might have on hand. Also, are you making the soup with beef or chicken stock?
Just trying to use what I have on hand. The recipe I'm using calls for both chx and beef stock.
If you're trying to go alcohol-free you can also substitute white wine vinegar. It adds a bit more acidity than the wine would. If you prefer a sweeter flavor then white cranberry or white grape juice works as well.
I have just written an article about this so check it out here: http://www.topfive.tk/2015/01/my-top-5-dry-sherry-substitutes.html